Though some education experts claim that the low test scores

[ivy]

Though some education experts claim that the low test scores of children in the city can be raised by hiring more teachers and an increase in the amount of funding for textbooks and other supplies, others insist that doing this cannot guarantee any improvement.
- by hiring more teachers and an increase in the amount of funding for textbooks and other supplies, others insist that doing this
- by hiring more teachers and increasing funding for textbooks and other supplies, others insist that doing so
- by a hiring of more teachers and increasing the amount of funding for textbooks and other supplies, other insist that doing so
- by hiring more teachers and increasing the amount of funding for textbooks and other supplies, others insist that doing this
- by hiring more teachers and an increase in funding for textbooks and other supplies, others insist that doing so

The answer is option B. The explanation says that 'doing so' is preferable to 'doing this' or 'doing it' as the verb phrase 'hiring more teachers and increasing funding for textbooks and other supplies' can not be replaced by pronouns 'this' or 'it'. Then how can 'so' replace the verb phrase 'hiring more teachers and increasing funding for textbooks and other supplies'? Which part of the island is 'so' from to have such a privilege? :P As far as i know, so can be used as an adverb or a conjunction.

Thanks very much! :)

[tim]

this is an idiom you just have to memorize. idioms generally don't have explanations, they just are the way they are..